Human retroviral diseases such as AIDS are among the most challenging medical problems facing biomedical science today. A better understanding of the genetic variation of retroviruses as well as the molecular details of virus-host cell receptor interaction will enable us to better predict the behavior of these viruses as well as make an impact on managing the diseases they cause. Avian retroviruses provide an excellent model system for studying these features of retroviral biology. The goals of this project are to examine how the avian retrovirus envelope (env) gene determines host range, as well as how viral evolution might lead to altered host cell receptor usage. Entry of retroviruses into cells involves interactions between viral envelope glycoprotein and host cell surface receptors. Viruses using the same receptor have a similar host range and exhibit infection interference with one another; these properties specify a viral subgroup. The basis for this host range specificity is incompletely understood. This study will focus on delineating specific sequences of the env gene which are responsible for subgroup specificity. Additionally, we will investigate how viral evolution might have allowed similar retroviruses to acquire the ability to use different receptors. Two avenues of inquiry will be pursued in parallel. Firstly, we will use the ability of retroviruses to evolve rapidly in response to selective pressure to force the selection and outgrowth of a virus with a mutant env gene that confers expanded host range. To do this we will passage virus on mixtures of permissive and non- permissive cells under conditions in which subtle selective pressure for expanded host range is exerted. The sequence of such mutant env genes will be examined to define the changes that lead to altered host range. Additionally, we will look at the kinetics of appearance of the mutant to explore the roles of mutation and selection in the evolution of receptor usage. Secondly, we plan to create a series of recombinant clones in which portions of env variable regions from members of different viral subgroups are joined. Examining the host range of viruses bearing these recombinant env genes will allow us to delineate small region(s) of env that are important in host range specificity. A mutagenesis procedure using degenerate pools of oligonucleotides encompassing portions of these regions will be used to generate virus carrying different arrays of mutations in this portion of the env gene. The host range of these mutated viruses will then be examined. The env genes of variant viruses with extended host range (able to infect "non-permissive" cells) and viruses which are still able to infect permissive cells (virus with preserved essential sequences) will then be cloned and sequenced to define the regions critical for host range variation. These studies will provide important insights into the mechanisms of host range variability of retroviruses, as well as the evolution of receptor usage. Such information may also permit a better understanding of the initial virus-host cell interaction in other retroviral induced disease states such as AIDS.